[snip, some previous and replies. Just to reply to onepoint]> > We plan on taking 50 data points from each school, about 20-25% of the> class size. So you say that we should take the same number of points> even if one school has more students than the other? Also, do you mean> that the sample size is insufficient to draw reliable conclusions> about whether coed schools really lessen the gender differences?>

The earlier poster tried to make this point, and I willtry again. Whether you are comparing 50 or 500students from two schools, you are only comparing*two* schools. Or six schools. Within the schools,there is a lesser hierarchy, perhaps, of a dozen teachers -- or whatever. If you see differences betweentwo schools, is it something idiosyncratic to thosetwo schools, or idiosyncratic to that selection of adozen teachers?

If you want draw an inference about "schools", it isalmost necessary to have a large number of *schools*represented. There is a statistical approach that tries totest across multiple strata, but that quickly gets complicated.

With two schools, or a few schools, you can easilydo comparisons, by t-tests or ANOVA, that tell you whetherthese particular schools seem to "differ". That can be astarting point for discussing all the *wrong* reasons thatmight exist that could account for the differences. Selective admissions? Selective attrition? Particularteachers?

Even when there are 50 or 500 *schools* being sampled,the resulting "inference" (whether coed schools differ) canbe challenged for systematic biases. With a large numberof schools, it begins to be possible to argue that variousfactors have been measured, and that they indeed do seemto be balanced or equivalent between schools, or otherwiseaccounted for.

With a few schools -- try to keep the questions simple, andtry to keep the conclusions well-tempered by doubts.